Tarantula infected with Cordyceps

Cordyceps is a genus of ascomycete fungi (sac fungi) that includes about 400 identified species and many yet to be described. All Cordyceps species are endoparasitoids, mainly on insects and other arthropods (they are thus entomopathogenic fungi); a few are parasitic on other fungi. The best known species of the genus is Cordyceps sinensis, first recorded as yartsa gunbu in Tibet in the 15th Century. It is known as yarsha gumba in Nepal. The Latin etymology describes cord as club, ceps as head, and sinensis as Chinese. Cordyceps sinensis, known in English commonly as caterpillar fungus, is considered a medicinal mushroom in oriental medicines, such as traditional Chinese medicines and traditional Tibetan medicine.

When a Cordyceps fungus attacks a host, the mycelium invades and eventually replaces the host tissue, while the elongated fruiting body (ascocarp) may be cylindrical, branched, or of complex shape. The ascocarp bears many small, flask-shaped perithecia containing asci. These in turn contain thread-like ascospores, which usually break into fragments and are presumably infective.

Some Cordyceps species are able to affect the behavior of their insect host: Cordyceps unilateralis causes ants to climb a plant and attach there before they die. This ensures the parasite’s environment is at an optimal temperature and humidity, and that maximal distribution of the spores from the fruiting body that sprouts out of the dead insect is achieved. Marks have been found on fossilised leaves which suggest this ability to modify the host’s behaviour evolved more than 48 million years ago.